Method of making a variable weight cable

ABSTRACT

A method of making a variable weight electrical cable includes steps of placing uniformly thick layers of materials of different density over a central core, including the method of changing from a foamed extrudate to a solid extrudate in a layer.

United States Patent Carlson et al.

METHOD OF MAKING A VARIABLE WEIGHT CABLE Inventors: Richard H. Carlson, Sterling Junction; Edward M. Felkel, Worcester, both of Mass.

United States Steel Corporation, Pittsburgh, Pa.

Filed: May 7, 1971 Appl. No.: 141,326

Related US. Application Data Division of Ser. No. 21,614, March 23, 1970, Pat. No. 3,605,398.

Assigne'e:

References Cited UNITED STATES PATENTS 11/1964 Garshick l74/101.5

/L CIRCULATOR In .122 76 74 g; 5 SPEED 5+ CONTROL II II EEEEEEiEEEE JHEEEEE [451 Apr. 23, 1974 2,725,713 12/1955 Blanchard 57/147 X 2,885,737 5/1959 Whalen 264/47 2,820,987 1/1958 Bunch 264/47 X 3,557,265 l/1971 Chisholm 264/47 3,605,398 9/1971 Carlson 57/149 X 2,518,454 8/1950 Elliott 264/47 X 3,064,073 1 1/1962 Downing... 264/47 X 3,411,981 11/1968 Thomas 264/47 X 2,994,327 8/1961 Otto 264/47 3,413,387 11/1968 Ohsol 264/46 2,403,693 7/ 1946 Urmston 174/ l 3,299,192 l/l967 Lux 264/48 Primary Examiner-Donald E. Czaja Assistant Examiner-H. S. Cockeram Attorney, Agent, or Firm-Rea C. Helm [57] ABSTRACT A method of making a variable weight electrical cable includes steps of placing uniformly thick layers of materials of different density over a central core, including the method of changing from a foamed extrudate to a solid extrudate ina layer.

4 Claims, 5 Drawing Figures RA T/O POTENT/OMETE/P ""82 SPEED CONT/L 80 PATENTEDAFREJ 1974 11806568 MIDDLE TRAIL l/VG Elva SECT/OIV SECTION 34 r M I? 3 llzr 22 if? a v 26 IIVVEN R5 REWARD h. 64 SUN EDWARD M FEL/(EL 8y M Attorney I LATENTEUAPR 2 3 m: 13.8 06; 56 8 sum 3 OF 4 FIG: 3

INVENTORS RICHARD H. CARL 8 EDWARD M. FEL

Attorney METHOD OF MAKING A VARIABLE WEIGHT CABLE This application, which is a division of our copending application Ser. No. 21,614, filed Mar. 23, 1970 now US. Pat. No. 3,605,398, relates to a method of making an electrical cable, and more particularly to a method of making a continuous length tow cable which has a uniform diameter and sections of different strength and weight in sea water.

In oceanographic research, off-shore oilwell exploration and sonar operations, sensors or other detection devices are used for locating undersea objects and phenomena. Sensors are positioned at a known distance from a ship and at a known distance below the ocean surface. Since the sensor must be maintained in fixed relationship to the ship, special towing cables of controlled weight and strength are required.

These cables require high strength and weight near the ship, but the trailing end of the cable has better hydrodynamic characteristics if it has little weight in sea water. Such cables have been produced by splicing two separate sections, a ship-end section that is heavy and strong and a trailing end section of considerably less strength and weight. This splice has been the cause of poor cable performance because of mechanical difficulties caused by the sudden change in cable weight and because of electrical difficulties caused by the ingress of moisture at the splice. In addition, the trailing end of the cable tends to pivot around the splice area resulting in fatigue and poor hydrodynamic characteristics.

In accordance with our invention, a variable weight cable is formed having layers with changing density, such as armor wires, extruded solid polyethylene and extruded foamed polyethylene.

It is therefore an object of our invention to provide a method of making. a cable having layers of uniform thickness and changing density.

Another object is to provide a method for extruding at a constant size while changing from a foamed extrudate to a solid extrudate.

These and other objects will be more apparent after referring to the following drawings and specification in which:

FIG. 1 is a fragmented longitudinal section of a variable weight cable.

FIG. 2 is a cross-sectional area of the ship-end section of the cable along line II--II of FIG. 1.

FIG. 3 is a cross-sectional area of the mid-section along line III-Ill of FIG. 1.

FIG. 4 is a cross-sectional area of the trailing end along line lVIV of FIG. 1; and

FIG. 5 is a schematic arrangement of apparatus for practicing the method of extrusion of our invention.

Referring now to the drawings, reference numeral 2 indicates a central conductor of six copper wires 4, surrounding a nylon monofilament 'core 6. Surrounding conductor 2 is a crystalline ethylene-propylene copolymer insulation 8. A copper wire braid I0 is applied over insualtion 8 as an outer conductor. A polyester film tape 12 is applied over braid and a crystalline ethylene-propylene copolymer jacket 14 is applied over the tape 12. While a conventional coaxial cable has thus far been described as the central core member throughout the entire length of our variable weight cable, other electrical constructions may be used depending on the requirements of each particular use.

' ence of the central core. The layers are preferably wrapped in opposite directions.

A solid polyethylene jacket 20, FIG. 4, is extruded v over layer 18 beginning from the trailing end of the cable to a first location 22, FIG. 1, where it is desired to change the weight and strength of the cable. A third layer 24, of twenty-four galvanized steel armor wires covering from about 93 to about 98 percent of the circumference, is helically wrapped over layer 18- as shown in FIGS. 2 and 3 for the length of the cable at the ship-end section and mid-section to location 22. Layer 24 is preferably wrapped in the opposite direction of layer 18, and is the same thickness as layer 20. This provides a relatively uniform diameter layer throughout the length of the cable, the armor wires 24 shown in FIGS. 2 and 3 and a solid polyethylene jacket 20 shown in FIG. 4. The trailing end 26, FIG. 4, is lighter, but not as strong as, mid-section 28, FIG. 3, and ship-end 30, FIG. 2.

A cellular polyethylene jacket 32 is extruded over jacket 20 and armor wire layer 24 beginning from the trailing end of the cable to a second location 34 where it is again desired to change the weight and strength of the cable. A fourth layer of galvanized steel armor wire 36, and a fifth layer of galvanized armor wire 38, FIG. 2, are helically wrapped over armor wire layer 24 beginning from location 34 to the ship-end of the cable. Layers 36 and 38 are each twenty-four wires covering from about 93 to about 98 percent of the circumference of the cable and together are the same thickness as jacket 32. Layer 36 is wrapped in the opposite direction of layer 38, and layer 36 is wrapped in the same direction of layer 24 to balance the torque created by the helically wrapped armor wire layers-l6, 18, 24, 36 and 38. This provides an additional relatively uniform diameter layer throughout the length of the cable, a cellular polyethylene jacket 32 shown in FIGS. 3 and 4, and armor wire layers 36 and 38 shown in FIG. 2. In this layer, the trailing end 26 and the mid-section 28 are lighter, but not as strong as ship-end section 30.

A cellular polyethylene jacket 40 is extruded over cellular jacket 32 in the trailing end 26, FIG. 4, and midsection 28, FIG. 3, to location 34 where a gradual change is made to a solid polyethylene jacket 42 of the same diameter for the balance of the cable, ship-end section 30, FIG. 2. This provides a third relatively uniform diameter layer throughout the length of the cable, a cellular polyethylene jacket 40 in the tralling end 26 and mid-section 28 which is lighter than the solid jacket 42 in the ship-end section 30. A solid polyethylene outer jacket 44 is extruded over jackets 40 and 42 throughout the length of the cable.

While armor wire layers l6, I8, 24, 36 and 38 are described as helically wrapped, braided armor wire may be used for one or more layers.

Building up the cable with uniform thickness concentric layers provides a cable free of splices and abrupt weight or strength changes which tend to weaken tow cables. The cable has a uniform diameter and is flexible enough throughout its length to be reeled and unreeled' over realtively small curvatures without weakening the cable. The length and thickness of solid polyethylene, foamed polyethylene and armor wire in the sections may be varied to provide any desired combination of weight and strength.

While the solid jackets and the armor wires are placed on the cable by conventional methods, the cellular jackets require particular methods of extrusion.

In order that cellular jackets 32 and 40 will meet requirements of specific gravity, a smooth surface, a closed-cell structure and low water absorption at high pressures, they should be made ofa foamable, low density polyethylene. The material used should have a high melt strength at the temperature of decomposition of the blowing agent. If melt strength is too low, the gas liberated by the blowing agent encounters little resistance to expansion and the result is a large open-cell structure with a rough surface. A highly branched, high molecular weight polyethylene provides a rubber-like consistency to the melt. This allows more latitude in processing temperatures because the melt viscosity does not decrease rapidly with increased temperatures. In addition, the extrusion must be made under carefully controlled conditions as hereinafter described.

Referring now to FIG. 5, extruder 46 is a conventional extruder used for extruding the cellular and solid jackets. Oil is circulated through lines 48 and 50 through screw 52 at a predetermined temperature by a circulating heating unit 54, such as a Sterlco Model 6016 unit manufactured by Sterling Inc., of Milwaukee, Wise. A pressure gage 56 measures the pressure of the melt in the crosshead S8. A noncontact, infrared radiation thermometer 60, such as a Model 300T] Ircon Radiation Thermometer manufactured by Ircon Inc., of Chicago, 111., is focused on the extrudate 62 just as it leaves dies 64 and 66 in crosshead 58. This method of temperature measurement avoids potential errors of measuring the temperature of a heated crosshead instead of the extrudate which is often the case in a crosshead thermocuouple. A water cooling trough 68 is located just beyond the crosshead 58, preferably no more than about 4 feet. A capstan 70 is located just beyond the cooling trough 68 and is driven by variable speed motor 72. Another variable speed motor 74 drives screw 52 through a gear reducer and transmission 76. Motor 72 and 74 are connected to a power source (not shown) with speed controls 78 and 80 respectively and a ratio potentiometer 82 to change the ratio of the speed motors 72 and 74. The finished cable is reeled on a conventional take-up 84. Electrical resistance heaters 86 surround screw 52 in four separately controlled heating zones 88, 90, 92 and 94. Electrical resistance heater 96 surrounds crosshead 58.

In extruding the cellular section of our variable weight cable. short-land, pressure-type dies 64 and 66 are preferred. Die sizes are selected by conventional methods. depending on the desired dimensions of the finished cable. the core diameter and the desired gas content. For example, cellular layers 32 and 40 have a 40 percent gas content which, when combined with the other cable elements, make a trailing end 26 nearly buoyant in sea water. Die spacing is set for a melt pressure between 300 and 1,000 psig. Insufficient head pressure allows blowing to occur inside the crosshead resulting in an open-cell structure with a rough surface, seriously diminishing a desired high hydrostatic pressure capability. Too high a head pressure restricts output and results in excessive expansion in the extrudate.

Temperature settings for a 40 percent gas content extrudate should be about l50F for the screw circulating oil, 40F. to 50F for quench water, 290F for zones 88, and 92, 300F to 330F for zone 94, 270F to 290F for crosshead 58 and 270F for dies 64 and 66. The extrudate temperature as read by thermometer 60 should be 325F to 330F.

In order to maintain both the desired extrudate temperature and the desired diameter, ratio potentiometer 82 is used to control the ratio of the speed of the screw 52 and the speed of the capstan 70. Screw speed is used for fine temperature control along with the ratio control.

The rate of cooling of the extrudate will also affect the specific gravity of the extrudate. The rate of cooling is set by the distance between the crosshead 58 and the cooling water trough 68, the water temperature and the line speed. When layer 32 is extruded, the difference in thermal capacity of armor layer 24 over plastic layer 20 requires a slightly higher extrudate temperature. Temperature changes at location 22 are accomplished by feed screw speed changes.

In extruding the layer including jackets 40 and 42, it is preferred to start with the cellular section 40. About feet before location 34 is reached, extrusion speed and temperature are reduced and the feed changed to a solid, low density polyethylene compatible with the foamable, low density polyethylene. Since reducing the temperature tends to reduce the diameter, the speed ratio is constantly adjusted so that diameter will remain constant during the transition. The diameter stabilizes when all the foamable material has been displaced and at this time the temperature profile of the crosshead is raised to that normally required for a solid, low density polyethylene jacket extrusion.

We claim:

1. A method of continuously extruding a concentric layer over a generally round central core member of a variable weight and strength and splice free cable comprising the steps of extruding through a die a foamable highly branched high molecular weight low density polyethylene to form a cellular layer of generally uniform thickness over said core member to a predetermined location on the length of said core member and without interruption continuing to extrude said layer to bring about a gradual change in the layer from cellular to solid by feeding to the extruder a solid low density polyethylene compatible with said foamable polyethylene to form a continuing layer of solid polyethylene of essentially the same thickness as said cellular layer.

2. A method according to claim 1 in which changing the layer includes the steps of reducing the extruder feed screw speed and changing the ratio of the extruder feed screw speed to the extruder line speed during the change period to lower extrudate temperature and maintain extrudate dimensions and changing extruder screw temperature profile, extruder crosshead temperature and extruder die temperature to that required for solid extrusion.

3. A method according to claim 1 which includes changing the extrudate temperature when the heat conductivity of the core member changes.

4. A method according to claim 1 which includes the steps of extruding a second polyethylene concentric layer over said core member to a second predetermined location on the length of said core member and applying a layer of armor wires of the same thickness as the second polyethylene layer over the remainder of the cable. 

1. A method of continuously extruding a concentric layer over a generally round central core member of a variable weight and strength and splice free cable comprising the steps of extruding through a die a foamable highly branched high molecular weight low density polyethylene to form a cellular layer of generally uniform thickness over said core member to a predetermined location on the length of said core member and without interruption continuing to extrude said layer to bring about a gradual change in the layer from cellular to solid by feeding to the extruder a solid low density polyethylene compatible with said foamable polyethylene to form a continuing layer of solid polyethylene of essentially the same thickness as said cellular layer.
 2. A method according to claim 1 in which changing the layer includes the steps of reducing the extruder feed screw speed and changing the ratio of the extruder feed screw speed to the extruder line speed during the change period to lower extrudate temperature and maintain extrudate dimensions and changing extruder screw temperature profile, extruder crosshead temperature and extruder die temperature to that required for solid extrusion.
 3. A method according to claim 1 which includes changing the extrudate temperature when the heat conductivity of the core member changes.
 4. A method according to claim 1 which includes the steps of extruding a second polyethylene concentric layer over said core member to a second predetermined location on the length of said core member and applying a layer of armor wires of the same thickness as the second polyethylene layer over the remainder of the cable. 